Digital watermarking is a process for modifying physical or electronic media to embed a machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by altering the host media signal. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting imperceptible watermarks in media signals are detailed in the assignee's co-pending application Ser. No. 09/503,881 and U.S. Pat. No. 5,862,260, which are hereby incorporated by reference. Watermarking techniques particularly adapted to graphic art and halftone images are set forth in U.S. patent application Ser. Nos. 09/074,034, entitled Methods and Systems for Watermark Processing of Line Art Images, 09/689,226, entitled Halftone Watermarking and Related Applications, and 60/263,987, entitled Halftone Primitive Watermarking and Related Applications, which are hereby incorporated by reference.
In watermarking applications and related literature, digital watermarks are classified as robust, fragile and semi-fragile. A robust watermark refers to a watermark that is designed to survive typical and even malicious processing of the watermarked signal that distorts the watermarked signal and makes it more difficult to reliably detect and read the watermark. A fragile watermark refers to a watermark where the watermark degrades in response to certain forms of processing like printing copying, scanning, compression, etc. Fragile watermarks are typically used in authentication application to detect tampering of a signal. Semi-fragile watermarks combine the concepts of fragile and robust watermarks. These types of watermarks are designed to survive certain types of processing like compression, yet detect tampering like cropping or swapping of signals. Fragile and semi-fragile watermarks may be used to trigger certain actions or control usage of the watermarked content when degradation of the fragile watermark is detected.
In digitally watermarking media signals, such as audio, still images and video, there are a number of challenges and trade-offs. One challenge is to embed the watermark so that it is sufficiently robust for the particular set of attacks anticipated for the application, while making sure that the watermark is sufficiently imperceptible for that application. For some applications, it is not possible to fully anticipate the types of processing that a media object will encounter, even before it is distributed. For example, a music track may be produced and distributed in a number of different formats (different compression rates, different compression codecs, different broadcast formats, etc.). Each of these formats may degrade or distort the watermark differently. In addition, the music track may be rendered using high fidelity audio equipment, or lower quality equipment, giving rise to different perceptual quality constraints. In particular, lower quality rendering enables the watermark to be embedded more robustly because perceptibility constraints on the watermark are less stringent. The same is true for video signals, like movies, television programming, advertisements, etc.
In the case of still images, an image may undergo transformations, such as compression, color conversion, halftoning, etc. before it is finally printed or rendered. Consider, for example, graphic art used in advertisements, packaging, and brochures. Such art imagery may include a collection of a raster images that are combined to form a final image. For a particular design project, the graphic artist creates a piece of graphic art for a customer, typically including a collection of constituent images in different formats. Some of the images may be line art, vector graphics, color halftone or color multi-level per pixel images (in color formats like RGB, CMYK or YUV). The entire image product is described in a job ticket that encapsulates the rendering functions to control the assembly of the constituent images and the printing process.
The customer may want to apply a watermark to the final image product for a variety of applications, such as inserting a customer identifier for tracking purposes, linking the image to the customer's web site, etc. There are two main problems, potentially inter-related. One problem occurs with the content flow and timing of adding the watermark flow. Another problem occurs with adding watermarks to vector graphics. The stage at which the watermark message payload and embedding parameters are defined may not always be the appropriate stage to embed the watermark in the host signal. One place to embed the message payload of the watermark into the graphic art is in the raster interface processing (RIP) stage. In this stage, the constituent images are assembled and converted to a particular halftone image format compatible with the printer. The halftone image format includes one or more color planes of pixel elements that specify the presence or absence of ink at corresponding pixel locations. The RIP stage usually occurs at the Pre-Press house or Printer, and requires the person with the most critical eye for color. In addition, this stage, by definition, results in a complete raster image. The watermark can be defined for vector graphics (or line-art), but is ultimately embedded in a raster image when printed with common modem equipment. The customer doesn't usually interact with the Pre-Press house or Printer, except to possibly proof the image. In addition, these locations are under terrible time and cost constraints and do not want to deal with inefficient and costly customer interactions. Finally, many graphic art pieces contain little or no raster sections; thus, the watermark cannot be added before the art is rasterized at the RIP stage. Despite the difficulty of watermarking prior to rasterizing for printing, it is often necessary to preview the watermarked final image product on a display screen, or desktop printer, which poses the problem of how to embed the watermark for previewing.
If the graphic artist has to add the watermark before the Pre-Press house or Printer, the graphic artist must rasterize the image. This causes two problems. First, the graphic artist must now deliver a file consisting of a large number of bits (i.e. size). Second, the graphic artist is not the best person to deal with the color management required to produce a quality image.
The difficulty is that the customer is already working with the graphic artist and wishes to define the contents of the watermark, but the watermark is ultimately embedded in the rasterized image in the Pre-Press house or Printer. A similar problem exists for other media types like audio and video, where the watermark payload is specified at a stage different than the most appropriate stage for embedding the watermark in the content.
If the image file is a vector graphic, whether rendered for printing as described above, or distributed electronically such as on the web, a participant such as the owner, may want to watermark the vector graphic. The participant wants that watermark to be embedded in the rendered image whenever the vector file is rendered, such as on a computer screen, possible within a wed browser or printer. This allows illegitimate copies, such as copies made with a print screen function, to be identified.
A method for controlling watermark embedding in a media object through the use of a watermark embedding command is described below. In the process of creating the media object, the method includes a watermark embedding command among a set of one or more rendering commands that specify how the media object is to be rendered. For example, certain media signal formats like PCL, PDF, or postscript for images, MIDI and structured audio for audio signals, and MPEG-4 and MPEG-7 for audio and video signals, include descriptors that control how a particular media signal is to be rendered. The watermark embedding command includes a combination of the following items: an identifier used to link to customer or related content information, the customer's web site or store, the intensity at which to embed the watermark, areas not to embed, batch processing options, printing preferences for images, watermarking embedding methods to use on different media types, formats, or different parts of the media object, and desired rendering quality.
The watermark embedding command enables the customer or creator to specify watermark message payload and embedding parameters and preferences, and enables the rendering device to embed the watermark appropriately for a particular rendering process. In the case of graphic art, the customer can preview the watermarked content on the graphic artist's monitor or inexpensive printer, which rasterizes the image for display, embeds the watermark in response to the command, and renders the watermarked image. In addition, the Pre-Press house or Printer, can add and modify the watermark without interacting with the customer, thereby saving time and money.
In general, the watermark embedding command includes the message payload to be embedded and rules or links to how to embed these bits. Thus, the watermark function is implemented according to the desired embedding method when the graphic art is rendered, such as on the screen, printed proofs or final printing plates.
This method is extended to other types of media objects, including audio or music tracks, video sequences, etc. Further features will become apparent with reference to the following detailed description and accompanying drawings.
The objects of watermark embedding commands include the following: watermarks can be embedded in rendering description content, such as vector graphics, MIDI, and structured MPEG audio and video. In addition, watermarks can be embedded at a time and location separate from where and when the watermark and content is rendered. This reduces costs by allowing proper interaction between the content owner and creators, who have different responsibilities and skills.